White light sources that utilize light emitting diodes (LEDs) in their construction can have two basic configurations. In one, referred to herein as direct emissive LEDs, white light is generated by direct emission of different colored LEDs. Examples include a combination of a red LED, a green LED, and a blue LED, and a combination of a blue LED and a yellow LED. In another configuration, referred to herein as phosphor-converted LEDs (PCLEDs), a single LED generates light in a narrow range of wavelengths, which light impinges upon and excites a phosphor or other type of emissive material to produce light having different wavelengths than those generated by the LED. The phosphor can include a mixture or combination of distinct emissive materials, and the light emitted by the phosphor can include broad or narrow emission lines distributed over the visible wavelength range such that the emitted light appears substantially white to the unaided human eye.
An example of a PCLED is a blue LED illuminating a phosphor that converts blue light to longer wavelengths. A portion of the blue excitation light is not absorbed by the phosphor, and the residual blue excitation light is combined with longer wavelengths emitted by the phosphor. Another example of a PCLED is an ultraviolet (UV) LED illuminating a phosphor that absorbs and converts UV light either to red, green, and blue light, or a combination of blue and yellow light.
Another application of PCLEDs is to convert UV or blue light to green light. In general, green LEDs have a relatively low efficiency and can change output wavelength during operation. In contrast to green LEDs, green PCLEDs, can have improved wavelength stability.
Advantages of white light PCLEDs over direct emission white LEDs include better color stability as a function of device aging and temperature, and better batch-to-batch and device-to-device color uniformity/repeatability. However, PCLEDs can be less efficient than direct emission LEDs, due in part to inefficiencies in the process of light absorption and re-emission by the phosphor.